A Subpage Action allows a user to create a Subpage that has its own selection of Actions. It is a great way to have more useful content without cluttering the main page.
You build a subpage just line you build an app - you add a series of actions.
The Actions you have added to the Subpage are displayed below the create Action buttons. You can rearrange their order by dragging their handle (=).
Reference the Action Formatting and Controls FAQ for a refresher on the controls that are common to Actions.